Performance of Ni-Based Catalysts with La Promoter for the Reforming of Methane in Gasification Process

Abstract

The deactivation of active sites caused by high-temperature sintering and the deposition of a large amount of carbon are the main difficulties in the reforming of methane using Ni-based catalysts. La, as a promoter, has an ameliorating effect on the defects of Ni-based catalysts. In this article, the mechanism of action of Ni-based catalysts with the introduction of the rare-earth metal additive La was reviewed, and the effects of La on the methane-reforming performance of Ni-based catalysts were examined. The physical properties, alkalinity, and activity of Ni-based catalysts can be enhanced by the use of the auxiliary agent La, which promotes the conversion of CH₄ and CO₂ as well as the selectivity towards H₂ and CO formation in the reforming of methane. The reason why the Ni-based catalysts could maintain long-term stability in the presence of La was discussed. Furthermore, the current state of research on the introduction of different amounts of La in the reforming of methane at home and abroad was analyzed. It was found that 2–5 wt.% La is the most optimal quantity for improving the catalyst activity and stability, as well as the CO₂ chemisorption. The limitations and directions for future research in the reforming of methane were discussed.

1. Introduction

Methane is a valuable raw material for the production of hydrogen, due to its abundance and high hydrogen-to-carbon mass ratio [1]. Hydrogen is a renewable and clean energy source with a high calorific value and good combustion performance, and it therefore plays an important role in achieving zero emissions. Achieving green transformation is important for our future [2]. Therefore, hydrogen energy must be rationally developed and utilized to promote green and sustainable development. It is of paramount importance to limit the production of coke during the reforming stage, while simultaneously increasing the yield of hydrogen and reducing the production of carbon dioxide.

All Group VIII transition metals have been reported to exhibit high methane-reforming activity [3]. Among these, precious metals such as Ruthenium, etc., are considered to be highly resistant to carbon accumulation, making them excellent catalyst carriers [4,5]. Nevertheless, the high cost of precious metals is not conducive to their widespread application in production industries. Ni-based catalysts with strong methane adsorption and C–H decomposition activities, as well as abundant resources and low cost, represent one of the most promising catalysts for methane reforming processes. However, their actual catalytic effects are limited by factors such as carbon deposition and catalyst deactivation, in addition to poor mechanical performance. Ewald et al. [6] demonstrated that CO₂ can diffuse into the Ni phase and react with H₂ to promote the reforming reaction, thus increasing the activity of the Ni-based catalysts. However, due to the high conversion of CO₂ and CH₄, the Ni-based catalysts were exposed to a H₂-rich reducing atmosphere, which accelerated the sintering of the Ni particles [7]. This, in turn, led to a decrease in the storage capacity of CO₂ and the deactivation of the Ni-based catalysts. Methane reforming includes the steam reforming of methane (SRM, Equation (1)) and the dry reforming of methane (DRM, Equation (2))

CH₄ + H₂O ⇌ CO + 3H₂       ΔH_298K = +206 kJ/mol

(1)

CH₄ + CO₂ ⇌ 2CO + 2H₂       ΔH_298K = +274 kJ/mol

(2)

The reforming of methane results in the generation of CO and H₂ as the primary gaseous products. In addition, by-products such as solid carbon (C(s)), water, and light hydrocarbons are also formed. Hence, the following reactions can also occur in addition to the main reactions:

Methane decomposition:

CH₄ ⇌ C(s) + 2H₂       ΔH_298K = +75 kJ/mol

(3)

Water–Gas Shift Reaction:

H₂O + CO ⇌ CO₂ + H₂      ΔH_298K = −41 kJ/mol

(4)

Boudouard reaction:

2CO ⇌ C(s) + CO₂       ΔH_298K = −172 kJ/mol

(5)

Steam reforming of carbon:

C(s) + H₂O ⇌ CO + H₂      ΔH_298K = +131 kJ/mol

(6)

Generally, methane reforming is performed at elevated temperatures, between 700 and 900 °C, which rapidly deactivates Ni-based catalysts [8]. Steam (S/C ratio of 1~2) can promote the gasification of carbon deposition by the water–gas shift reaction and reduce the carbon deposition resulting from CO disproportionation at temperatures lower than 650 °C [9], but it may also cause sintering at high temperatures. The agglomeration between the carbon particles results in the formation of larger particles, which reduces the surface area and activity of the catalyst. Additionally, coke formation is inevitable at high pressures (7–28 bars) for almost all Ni-based catalysts [10]. The level of soot reduction decreases with increasing pressure [11]. The formed soot is more likely to be adsorbed on the catalyst surface and gradually aggregate, leading to sintering.

The structural properties of Ni-based catalysts, including particle size, also influence both the catalytic activity and the stability of the reforming reaction. Vogt et al. [12] demonstrated a correlation between the mean particle size of nickel and the formation of whisker carbon via TGA-MS experiments. The optimum Ni particle size for both the steam reforming of methane (SRM) and dry reforming of methane (DRM) was found to be approximately 2–3 nm at 500 °C, 600 °C, and 5 bar. The whisker carbon formed in SMR was observed to occur most frequently at a size of approximately 4.5 nm, whereas an increase in particle size led to an increase in whisker carbon in the case of DMR.

Whisker carbon mainly originates from the polymerization reaction between carbon atoms. A large amount of whisker carbon will block the active sites of the Ni-based catalysts, hinder the interaction between the material and the Ni-based catalysts, and alter the surface structure of the Ni-based catalysts. This will result in a reduction in the activity of the catalyst or even its deactivation. Therefore, the addition of promoters to Ni-based catalysts is considered to improve the performance of Ni-based catalysts in reforming reactions. It has been reported that the introduction of alkali metals (such as Na, K [13,14]), alkaline earth metals (such as Mg [15], Ca [16]), and lanthanide oxides (such as La₂O₃ [17], CeO₂ [18]) can enhance the interactions between the metals and the carriers, thereby reducing carbon deposition and improving the activity and stability of the catalyst.

La promoters not only enhance the oxidation–reduction capacity of Ni-based catalysts and provide more active sites but also regulate the surface acidity and alkalinity of the catalysts and improve the adsorption and reactivity of hydrocarbons, thus improving the anti-carbon deposition and recycling performance of the catalysts. Alotaibi et al. [19] studied the catalytic evaluation for Ni/ZL(B) (a zeolite material) catalysts with different promoters, and found that the rate of deactivation of LaNi/ZL(B) catalysts decreased at a slower rate, and the stability was higher than that of calcium promoters. Furthermore, it was demonstrated that La promoters could regulate the surface acidity of the carrier and inhibit catalyst coking, thereby enhancing the time-on-stream stability. Charisiou et al. [20] found that the enhancement of DRM catalytic activity of Ni-based catalysts with La promoters at 500–800 °C was mainly due to the better Ni dispersion and the increase in the concentration of the surface labile and mobile lattice O²⁻ ions. The former two can increase the number of active sites, facilitate the interaction between materials and Ni-based catalysts [21], and reduce the interactions between Ni particles, thus reducing carbon deposition and particle clustering. The latter regulates the redox reaction on the surface of Ni-based catalysts, which maintains the active sites and enhances the time-on-stream stability. This indicates that the Ni/LaZr catalysts have superior anti-sintering properties. This is mainly attributed to the protective action of a [O^δ−, δ+] dipolar layer created on the surface of Ni particles via spontaneous thermally driven spillover of O²⁻ ions from the support, by means of the repulsive electrical forces developed between neighboring particles. Furthermore, it was observed that the carbon content on the Ni/LaZr catalyst increased by approximately 20% at 550 °C compared to that on the Ni/Zr catalyst. This was attributed to the promotion of methane cracking and the Boudouard reaction by the La promoter. The presence of Ni²⁺ and Ni³⁺ oxidation states resulted in the formation of a perovskite oxide with lanthanum oxide (La₂O₃), leading to the generation of oxygen vacancies. The Ni-based catalyst, which was composed of lanthanum oxide, exhibited an excellent oxygen storage capacity due to the large number of oxygen vacancies and the La³⁺ redox potentials [22]. As the temperature increased, the redox properties of the La promoter enhanced the surface oxygen mobility and accelerated the carbon oxidation reaction, resulting in a decrease in the amount of coke and an increase in the proportion of easily oxidized amorphous carbon. Furthermore, the La promoter facilitated alkalinity on the Ni-based catalyst surface. The basic sites expedited the activation of CO. And the carbon and O⁻ basic sites can produce CO, which minimizes the amount of deposited carbon through CO disproportionation [23,24] and increases the stability of the catalyst.

In view of the deficiencies of Ni-based catalysts, the objective of this paper is to examine the influence of La on the reforming of Ni-based catalysts, focusing on the particle size, basic sites, and catalyst performance. This study provides a reference for experimental studies and theoretical analyses aimed at enhancing the activity and stability of Ni-based catalysts, as well as the selection of an appropriate La additive for the dry reforming of methane (DRM), steam reforming of methane (SRM), and related processes.

2. Mechanism of Reforming of Methane on the Ni-Based Catalysts with La Promoter

2.1. Particle Size Distribution and Reducibility

Various catalysts can be used for the reforming of methane, including natural mineral, alkali, and alkaline earth metal catalysts, as well as transition metal catalysts. Rare metals and Ni-based catalysts have been extensively studied for the reforming of methane to produce hydrogen [25]. Compared with precious metals, which are expensive and scarce, Ni-based catalysts are promising due to the abundance and availability of the raw materials, along with their low prices and high initial activity [26]. However, catalysts with a single Ni atom produce carbon deposits and ash during the reaction, which then accumulate on the surface of the Ni catalyst, reducing the number of catalyst active sites and the catalytic activity. In addition, the sintering of catalyst-active components at high temperatures can cause problems such as a decreased catalyst lifetime and hydrogen production rate. Lu et al. [27] found that even if the carbon accumulated on the catalyst surface is removed by high-temperature calcination and the catalyst is reduced with hydrogen, the activity of the used catalyst can only be partially recovered. This is due to the fact that the Ni-based catalyst is prone to agglomeration, sintering, and poor dispersion, which results in accelerated deactivation of the catalyst and subsequent poor hydrogen production. Consequently, it is of critical importance to enhance the dispersion, carbon deposition resistance, and stability of the active component in Ni-based catalysts.

An effective way to improve the anti-coking accumulation and anti-carbon deposition performance of Ni-based catalysts is to include additives. Additives can regulate the interaction between the catalyst and carrier, inhibiting the crystallization and agglomeration of the catalyst, to improve the dispersion of metal active components and the carbon deposition resistance of the catalyst while improving the service life and stability. During the reforming of methane, Ni-based catalysts with additives exhibit better catalytic performance than those without additives. Common auxiliaries include alkali, transition, and rare-earth metals. Alkali metal catalysts are typically presented in the form of metal salt compounds loaded onto a support. The cleavage of aliphatic compounds and oxygen-containing functional groups can be significantly promoted by alkali metals [28], including K, Ca, and Mg, while simultaneously enhancing the anti-carbon deposition ability, thus improving the catalytic activity and stability. The introduction of transition metals forms x-Ni (x = Co, Fe, Mo, etc.) alloys that reduce particle aggregation and significantly improve metal dispersion, thereby improving the carbon deposition resistance of the catalyst and prolonging its service life. The chemical activity of rare-earth metals is considerable, and they can significantly suppress catalyst deactivation, improve the catalyst stability, and promote catalytic effects. The most commonly used rare-earth metals include La, Ce, Pr, and Ru. Lv et al. [29] found that the reduction temperature of Ni-based catalysts could be decreased by the introduction of an appropriate amount of auxiliary Ru into the Ni-based catalyst. The Ni-based catalyst exhibited a strong interaction with Ru, which improved the dispersion of metallic Ni and promoted the formation of intermediates with high activity, preventing the rapid inactivation of the catalyst. As an oxygen carrier, Ce reduces the crystal size of Ni and increases Ni dispersion, thereby increasing the active surface area [30]. Moreover, it can improve the active components of the catalysts owing to its characteristics, thus promoting the reforming of hydrogen. Zeng et al. [31] studied the effects of Mg, Mn, and La additives on the catalytic performance of Ni-based reforming. They found that the catalysts containing La exhibited smaller Ni particles and better CO adsorption performance after reduction. However, the addition of the alkaline earth metal Mg and the transition metal Mn did not weaken the interactions between the active components and the carrier. Previous research [32,33] has demonstrated that the particle size of the active component, nickel, significantly affects the activity and stability of the catalyst. Smaller Ni particles result in more uniform dispersion, which improves the anti-carbon deposition performance of the catalyst. The critical size of Ni for carbon deposition ranges from 3.0 to 10.0 nm, and smaller Ni particles result in lower carbon deposition rates. However, from the perspective of anti-sintering, a Ni particle size that is too small or large degrades the anti-sintering ability of the catalyst. Theoretically, when the Ni particle size is approximately 6.0 nm [34], the anti-sintering ability of the catalyst is optimal.

Figure 1b–d present the average Ni particle size of Ni with alkali (Mg), transition (Co), and rare-earth (Ce) metals at approximately 12.4, 10.0, and 9.3 nm, respectively. As shown in Figure 1a, among the Ni-based catalysts with La, Ni particles with a size of 6.0 nm exhibited the highest frequency, and the average Ni particle size was smaller, i.e., approximately 9.2 nm, indicating that the addition of an appropriate quantity of La can facilitate the dispersion of the active component Ni while simultaneously inhibiting the high-temperature aggregation of Ni particles. A homogeneous particle size distribution is beneficial for inhibiting catalyst sintering, thereby preventing a decrease in catalyst activity with an increase in particle size during the catalytic process. Furthermore, La facilitates the reducibility of NiO in the catalyst by weakening the interaction between NiO and Al₂O₃, thus making it easier to transition from the oxidized state to the active metal state. Song et al. [35] found that the binding energy (BE) of metallic Ni exhibited a negative shift from 852.7 to 852.3 eV in the presence of La, indicating that La donated electrons to Ni due to the electropositive nature of La. This resulted in Ni becoming more electron-rich. Furthermore, the higher capacity of La to bind with electrons than Al suggests that Ni atoms surrounded by La became more dominant than Ni atoms surrounded by Al, which resulted in a weakened interaction between NiO and Al₂O₃ due to the addition of La. This effect can help disperse the NiO particles and increase the accessibility of their surface active sites. Zhang et al. [36] observed the crystalline phase structure of Ni-based catalysts before and after reduction by X-ray diffraction (XRD), as shown in Figure 2. From Figure 2a, it can be observed that the intensity of the NiO characteristic peaks of Ni-based catalysts with lanthanide metal oxides is less pronounced than that of Ni/mesoporous silica spheres (MSSs). This indicates that lanthanum metal oxides prevented the NiO particles from agglomerating and thus improved the dispersion of the active components. From Figure 2b, it can be concluded that lanthanum metal oxides resulted in a greater number of Ni particles within the pores of Ni-based catalysts, which were less prone to oxidative reactions and reduced the possibility of forming carbon deposits [37].

Wang et al. [42] analyzed the TEM of reduced (Figure 3) and used (Figure 4) catalysts and found that compared to Ni/SBA-15 (Figure 3a,d), in the presence of a La promoter (Figure 3b,c), Ni was evenly dispersed on the surface of SBA-15, which significantly reduced the agglomeration of Ni during the reduction process. This was attributed to the stronger interaction between Ni and oxygen-containing functional groups. It was also found that small-sized Ni particles were evenly distributed on the surface of the Ni-La₂O₃/SBA-15(C) catalyst. The Ni-La₂O₃/SBA-15(C) catalyst was prepared by a citrate complex method with 10 wt.% Ni. A mixture of citric acid and ethylene glycol was added to the Ni (NO₃)₂·6H₂O and La (NO₃)₂·6H₂O solution. The complexes formed by ions were dried at 120 °C for another 12 h, and then calcined at 350 °C for 2 h, and maintained at 700 °C for 5 h. During the process, a key precursor was formed—LaNiO₃ with a perovskite structure. The formed LaNiO₃ precursor exhibited excellent structural stability and indeed played an important role in promoting the dispersion of Ni active sites and limiting the growth of Ni particles, whereas Ni-La₂O₃/SBA-15(I) had the same materials added as those used in the citrate complex method, yet without citric acid and ethylene glycol. And the dried sample was calcined at 500 °C for 5 h. During the process, no perovskite structure was formed. So, the Ni-La₂O₃/SBA-15(C) catalyst showed a better reforming performance. When the Ni active sites maintain a high dispersion and have small particle sizes, the carbonaceous structure around the metal sites is easily exhausted and less prone to accumulation and coke formation [43]. This improves the reforming performance of the catalysts. Catalysts obtained by different preparation methods have different structures. Generally, the catalysts with higher porosity and greater interaction with the support metal exhibit superior performance. This makes it possible to obtain a more even distribution of the metallic phase, which in turn reduces the formation of harmful carbonaceous species that can deactivate the catalyst and cause sintering of the metal particles. This results in excellent catalytic stability and high conversion rates [44,45], whereas the formed perovskite phase may decompose the catalyst into the Ni⁰/La₂O₃ form during reduction [46]. The metallic Ni then activates the C–H bond, leading to the dissociation of CH₄ into CH_x and H_x species. The CH_x species also interacts with the mobile lattice oxygen produced at the metal–support interfacial region [47]. Finally, these species break down to form syngas, and the catalysts show superior activity.

Figure 4 shows transmission electron microscopy (TEM) images of the catalysts after use. It can be clearly seen that the particle size of Ni on the surface of Ni/SBA-15 (Figure 4a) was much larger than that of reduced Ni (Figure 3a), which was mainly due to the aggregation of Ni. Conversely, the particle size of Ni in the Ni-La₂O₃/SBA-15(C) catalyst (Figure 4c) remained smaller than that of the other catalysts. Therefore, the even dispersion of Ni and the presence of La inhibited the growth of Ni particles, which improved the sintering resistance of the catalyst [48]. From the H₂-TPR (Figure 5a), it can be seen that all catalysts exhibited a reduction peak at approximately 340 °C, which was attributed to the reduction of NiO combined with a relatively weak metal–support interaction. Ni-La₂O₃/SBA-15(C) showed a mild peak at 691.7 °C, which was mainly the reduction of NiO with a relatively strong metal−support interaction, ultimately generating smaller-sized and more active Ni particles, and thus delaying the agglomeration. It has been reported [49] that a high reduction temperature results in a stronger interaction between the metal and the carrier. The stronger the interaction between the metal and the carrier, the more difficult it is to reduce the catalyst and the better the dispersion of the metal on the catalyst surface will be [50]. It was also found that the reduction temperature of Ni-La₂O₃/SBA-15(I) was higher than that of Ni-SBA-15. This indicated that [51] La₂O₃ enhanced the interaction between the metal and the carrier and resisted sintering. Furthermore, the La promoter was found to increase the amount of H₂ consumed for the reduction of NiO from 1250 μmol/g to 1596 μmol/g, which suggested that more Ni particles were reduced, which was favorable for improving the catalytic activity.

Song et al. [52] found that the reduction peak at 440–460 °C shifted to a lower temperature on the Ni/La-COM catalyst. And the reduction peak area at 600–650 °C became smaller on the Ni/La-COM catalyst compared with the Ni-COM catalyst via H₂-TPR. This indicated that La could facilitate the reducibility of Ni species and the dispersion of Ni particles, resulting in increased Ni active sites. La enhances the interaction with Ni and the activation of CO₂ to gasify the surface carbon. Too high a CO₂ oxidation potential can cause the reoxidation of Ni, which can lead to the formation of inactive NiO. This, in turn, can cause a gradual loss of conversion. However, the reoxidation of Ni may be beneficial in limiting the size of agglomerated Ni particles by partially stabilizing the oxides on the support during the reaction [53]. La can slow down the encapsulation of active sites via carbon accumulation while enhancing the catalyst’s ability to activate and improving its stability. This results in a reduction in high-temperature sintering and agglomeration, in addition to an increase in H₂ production. The interaction between La and Ni significantly changes the oxidation and reduction properties of the catalyst, thereby influencing its overall performance. Bahari [54] introduced La into a Ni/Al₂O₃ catalyst via wet impregnation, which facilitated NiO reduction. In addition, the oxide form of the auxiliary La has good redox performance, which is conducive to the reaction. The storage and release of oxygen can oxidize carbon deposits on the surface of the catalyst.

2.2. Basic Sites

The surface properties of Ni-based catalysts in the reforming process are more acidic, which renders them susceptible to reaction with carbonaceous substances, leading to the formation of deposits. La, a rare-earth metal, has the capacity to modulate the acidity of Ni-based catalysts, while the char is more easily vaporized into gaseous products. And the char is more easily vaporized into gaseous products. The introduction of alkaline substances can neutralize these acidic sites, and the char is more easily converted into gaseous products.

Su et al. [55] studied the effects of La as a promoter on the properties and catalytic performance of Ni/Al₂O₃ in the supercritical water gasification (SCWG) of food waste and found that the alkalinity of La promoted the dissociative adsorption of CO₂, thereby inhibiting carbon formation by generating more oxygen atoms near the catalytic active metal surface. The La promoter improved the catalytic activity of the catalyst, thereby extending its lifetime. Wang et al. [42] also found that an increase in β and δ leads to peaks in the range of 150~200 °C and 300~350 °C, respectively, from the CO₂-TPD analysis, indicating the presence of weak and medium basic sites, respectively, and a decrease in the number of strong basic sites compared to xNi/SBA-15 (Figure 5b) in the presence of La. In particular, weak basic sites related to surface OH⁻, medium basic sites related to metal–oxygen pairs, and strong basic sites related to low-coordinated surface O²⁻ [56]. α and β are mainly due to the formation of bicarbonates by the surface OH species adsorbing CO₂, which is related to the weak basic sites. δ is mainly due to the presence of metal (Ni²⁺, La³⁺)–oxygen (O²⁻) pairs with moderate intensity. γ is mainly the interplay of lattice oxygen species and CO₂ [57]. This indicates that La can increase the number of metal–oxygen pairs (La-O) on the catalyst surface, leading to an increase in the number of medium basic sites [58], which is beneficial for CO₂ adsorption and activation. Moreover, the surface alkalinity of the catalysts was also affected by the carrier structure [59]. The presence of La resulted in the active components of Ni-based catalysts being evenly dispersed on the surface, which increased the number of active sites on the surface, including the basic sites. Medium alkalinity led to an optimal CO₂/carbonate balance [60], which provided a continuous supply of O₂ on the metal surface and prevented catalyst deactivation due to serious char deposition. Li et al. [61] also drew the same conclusion.

2.3. Mechanism of Reforming of Methane on the Ni-Based Catalysts with La Promoter

The sintering and carbon deposition in the active components of metal particles are generally considered to be the main causes of catalyst deactivation in the reforming reaction. Carbon deposition is primarily caused by the cracking of methane on the active center and the disproportionation of the reaction product CO [62], which blocks the pores of the catalyst or covers the active components of the catalyst, leading to the deactivation of the catalyst. The following two reactions [63] are the main causes of carbon deposition over Ni-based catalysts in the reforming reaction:

CH₄ = C + 2H₂

(7)

2CO = C +CO₂

(8)

During the reforming reaction, Ni-based catalysts are prone to forming Ni clusters, while inactive carbon tends to form on the catalyst, leading to the deactivation of the catalyst due to blocked pores and wrapped active sites. Typically, Ni-based catalysts form whisker carbon at high temperatures. Whisker carbon is the most widely observed form of carbon; however, when too much carbon accumulates in this form, it can block the reforming reaction tube [64], terminating the reaction. In fact, the mechanism of coke formation and the changes in its properties play a crucial role in the deactivation process [65]. As the temperature increases, the yield of whisker carbon gradually decreases, and its structure becomes denser. This results in the formation of non-filamentous coke. The non-filamentous coke is coated with carbon, which is the most destructive product of the reforming reaction. This is the main reason for deactivation caused by Ni-site plugging under these conditions [66]. Carbon deposition encapsulates the active metal Ni in the catalyst, reducing the number of reactive sites in the reaction and hindering the activation and dissociation of CH₄ and CO₂ at the active sites. CH₄ dissociates on the surfaces of the Ni particles, forming H₂ and activating carbon intermediates [67,68].

With excessive carbon accumulation, the carrier pores are severely blocked, which not only leads to the deactivation of the active component but also destroys the overall structure of the catalyst, leading to the collapse of the catalyst structure [69] and the blockage of the catalyst bed. Thereafter, the catalyst is inactivated to a high degree, and the coke formation is slow as long as the Ni sites remain blocked. Finally, pyrolytic carbon is formed. Generally, this form of carbon appears in acidic catalysts and causes their complete inactivation [70]. The sintering of active metal particles of Ni includes atomic and microcrystal migration [71], and these sintering mechanisms occur simultaneously during the catalytic reaction. Generally, the sintering of a metal is irreversible, and the sintering rate increases exponentially with an increase in the temperature. During the catalytic reaction, the active metal Ni is oxidized and reacts with the support or auxiliary agent. The pore structure of the catalyst support collapses at high temperatures, which also causes deactivation of the catalyst [72,73]. A larger specific surface area implies a higher degree of dispersion of the active metal, a smaller particle size of the active Ni particles, higher catalytic activity, and higher conversion rates of the catalyst for CH₄ and CO₂. Reducing the Ni grain size can significantly improve the activity and selectivity of the catalyst, indicating that the deactivation of the catalyst occurs primarily due to the formation of large amounts of carbon deposits [74]. However, the interfacial area among carbon, La₂O₃, and Ni⁰ can result in the formation of synergistic sites, such as basic sites, which enhance the resistance to carbon deposition [75]. The addition of La by different preparation methods leads to the formation of metal oxides such as La₂O₃ on the surface of the Ni-based catalyst. The decrease in particle size of the active component, NiO, facilitates its distribution on the carrier surface, preventing sintering. The diminution of the Ni grain size may be ascribed to the diffusion of carbon intermediates and products through the Ni particles, thereby forming Ni-C, which is considered to be an intermediate product in the formation of filamentous coke. As the reaction proceeds, the filamentous coke or whisker carbon gradually separates from the Ni crystals by dragging the Ni particles as they grow. However, no blocked sites are present, and thus, no discernible effect is observed on the activity of the catalyst. Ni-based catalysts with La exhibit superior coking resistance and more stable catalytic performance. The hydrophilic oxide La can also facilitate the activation of adsorbed water, promote coke gasification, enhance the reduction of the catalyst, reduce the particle size of Ni metal, and influence the CO₂ conversion and CH₄ selectivity of the catalyst [76]. Furthermore, La enhances the adsorption of CO₂, regenerates the Ni surface, reduces coking, and improves stability under reforming reaction conditions. During the reforming reaction, CH₄ is primarily decomposed on the Ni crystal to form H₂ and surface carbon, whereas CO₂ is adsorbed onto the La₂O₃ support to form La₂O₂CO₃ [77]. At high temperatures, the reaction of La₂O₂CO₃ with the carbonaceous material formed on the surfaces of the Ni crystals results in the production of CO and the restoration of the original state of the Ni surfaces [78]. CO₂ is adsorbed onto La₂O₃ to form La₂O₂CO₃, which can react with the adjacent active carbon intermediates to form CO and regenerate La₂O₃, thus completing the recycling of La₂O₂CO₃ and La₂O₃ [79]. When Ni metal is well dispersed, La₂O₂CO₃ participates in decoking, preventing the accumulation of La₂O₂CO₃. The introduction of La also improves the basic performance of the oxygen carrier and enhances the adsorption capacity of CO₂. La oxides participate in the adsorption and activation of CO₂ [80], and the corresponding reaction equations are as follows:

La₂O₃ + CO₂ → La₂O₂CO₃

(9)

La₂O₂CO₃ + Ni-C → La₂O₃ + 2CO + Ni

(10)

CH₄ + CO₂ → 2CO + 2H₂

(11)

The use of La₂O₃ carriers that have been loaded with Ni metal can cause the catalyst to form a perovskite, in which Ni is well dispersed. This can relieve the degree of coverage of the active metal components by carbon deposition and reduce the deactivation rate of the catalyst [54]. Moreover, the La₂O₃ carrier is alkaline, which is beneficial for the adsorption of CO₂ to form La₂O₂CO₃ [81]. This consumes some of the carbon accumulated in the product while improving the catalyst activity (Figure 6 and Figure 7).

3. The Effects of La Addition on the Reforming of Methane in Ni-Based Catalysts

In the Ni-based catalysts with La, the C–H bond is likely to be polarized, activating methane, which is the primary reason for the improved reforming performance of La-modified Ni monolithic catalysts. Over-polarization of the C–H bond of methane may lead to a shift in product selectivity, generating undesired by-products or olefins. Quindimil et al. [82] found that the addition of La₂O₃ to Ni–zeolite catalysts promoted the dispersion of Ni particles into smaller sizes and enhanced the CO₂ adsorption capacity of the zeolite. The surface basicity and dispersion of the Ni were enhanced, resulting in the formation of more basic and active sites, which can adsorb CO₂ and subsequently hydrogenate it into CH₄. When La is introduced as a Ni-based catalyst additive, it is imperative to strictly control the amount of the auxiliary agent La in order to promote catalysis. Ranjbar et al. [83] impregnated a 5 wt.% Ni/Al₂O₃ catalyst with an appropriate amount of lanthanum, which was introduced to enhance the CO selectivity and catalytic activity of this catalyst. It was revealed that the catalyst displayed a mesoporous structure and exhibited remarkable catalytic activity in the reverse water gas shift (RWGS) reaction. The addition of La resulted in an increase in the CO₂ conversion of 5Ni/Al₂O₃ from 39.8% to 43.2% at 700 °C. The introduction of La also improves the lifetime of the catalyst because the La₂O₃ can inhibit the sintering of the catalysts. Zhou et al. [84] found that the CO₂ conversion on a NiAl catalyst decreased from 79.2% to 65.1% after 120 h testing at 250 °C. In contrast, NiAlLa exhibited superior stability, with a CO₂ conversion decrease from 89.5% to 88.4%, by only 1% under the same conditions. This indicated that the presence of La₂O₃ greatly improved the catalytic stability of NiAl catalysts. Ha et al. [73] found that among all the studied catalysts, La-Ni/MgO-Al₂O₃ demonstrated the highest hydrogen production rates (118 L/(gcat·h)) at a reaction temperature of 800 °C, and effectively maintained them at a high and stable reactivity level during 160 h on stream. A small amount of La promoter does not cause any changes in the carbon deposition morphology or the reaction [85], while the excessive addition of La can be counterproductive. The presence of excessive La oxide reduces the concentration of Ni on the catalyst surface, as too much La covers the Ni atoms, preventing the reactants from fully contacting the active center [86], which degrades the overall performance of the catalyst. Barzegari et al. [87] studied the syngas production via propane steam reforming with La₂O₃-modified NiO-MgO-SiO₂ catalysts and found that Ni-based catalysts with 3 wt.% of La oxide could maintain a good structure, enhancing catalytic reduction and metal–support interaction while significantly improving the catalytic performance and maintaining a high level of catalytic activity. The presence of La₂O₃ improved the resistance against coke formation because of the formation of well-dispersed Ni species and the acceleration of the reaction through the La₂O₂CO₃. The introduction of excess La resulted in the accumulation of a considerable quantity of carbon on the catalyst surface. The catalyst channel was found to be blocked, resulting in a decrease in catalyst activity, which was more evident for 6 wt.% La₂O₃ loading. Huang et al. [88] found that the presence of La in the Ni-based catalyst promoted the distribution of metallic Ni over the Al₂O₃ substrate via XRD, thereby enhancing its activity. Nevertheless, with a further increase in Ni loading, the pore size distribution shifted from larger pores to narrower pores (approximately 5 nm), resulting in a decrease in specific surface area, pore volume, and size, which had a negative impact on the activity. Song et al. [89] found that an increase in the La mass fraction resulted in the formation of defective structures in the Ni/Al₂O₃ lattice, accompanied by an increase in dispersion and the specific surface area of Ni. This, in turn, weakened the affinity of the Ni catalyst for ethanol and affected the catalytic reforming of Ni to produce hydrogen. Similar findings were reported by Guo et al. [90]. When the amount of La exceeded a specific range, the crystallinity increased, resulting in poor dispersion of NiO on the surface of the carrier, as well as fewer strong alkaline sites; thus, Ni particles were oxidized, and the catalyst activity was reduced. Furthermore, Wang et al. [91] found that excessive La₂O₃ impeded the interaction between NiO and the carrier, inhibiting the migration and aggregation of Ni particles. This significantly affected the subsequent reforming. The effect of the appropriate amount of La added on the performance of the Ni-based catalyst is presented in

Table 1. The effect of La content on the properties of the catalyst.

Methods	Temperature (°C)	Amount of La (wt.%)	Effects	Reasons
Impregnation	800	3.0 [92]	CH4 conversion rate: 98.3%; CO selectivity: 82.5%; BET specific surface area: 137.5 m2·g−1	The C–H bonds of CH4 were more easily polarized with a more even dispersion of Ni on the catalyst surface.
Impregnation	800	2.0 [93]	H2 production rate: 9.70%; BET specific surface area: 98 m2·g−1	The grain size was reduced to enhance the dispersion of Ni on the Al2O3 carrier.
Impregnation	800	3.0 [94]	Maximum BET specific surface area: 97 m2/g	The dispersion of metal particles was improved, and the size of Ni particles decreased.
Impregnation	800	2.0 [95]	H2 conversion rate: 9.67%; BET specific surface area: 98.05 m2·g−1	The BET had a high specific surface area and a low grain size, which ensured the presence of a catalytic active region.
Impregnation	800	5.0 [96]	Smaller and well-dispersed nanoparticles of about 5–10 nm	The enhanced alkalinity and strengthened metal–support interactions led to an increase in the activity of the Ni catalyst.
Impregnation	700	3.0 [53]	BET specific surface area: 202 m2·g−1; the highest initial activity	The La provided abundant active sites, resulting in excellent stability.
Sol-gel method	700	4.0 [97]	BET specific surface area: 295 m2·g−1; the highest degree of Ni catalyst dispersion	The smallest Ni average diameter promoted catalytic effects.
Impregnation	700	4.0 [98]	BET specific surface area: 178 m2·g−1; CH4 conversion: 81.5%; CO2 conversion: 86.9%	The removal of carbon was accelerated by chemical adsorption and deionization of CO2.
Sol-gel method	700	3.0 [99]	BET specific surface area: 204 m2·g−1; conversion rates of CH4 and CO2 increased significantly.	The La promoted the dispersion of Ni, thereby enhancing the activity of the catalyst.

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The introduction of an appropriate amount of La improves the alkalinity of ordered mesoporous Ni-based catalysts. This enhances the adsorption of CO₂, inhibits carbon deposition, reduces the particle size of the active Ni metal in the catalyst, and improves the activity and stability of the catalyst. However, because of the hydrophilicity of La₂O₃, an excess of La may migrate to the surface of Ni-based catalysts, resulting in catalyst deactivation. Therefore, the La content must be controlled. As demonstrated in Table 1, the introduction of 2–5 wt.% La significantly improved the catalyst activity and stability, as well as the adsorption of CO₂.

4. Stability Performance of La Addition in Ni-Based Catalysts for Reforming of Methane

La promoters show better methane reforming performance over Ni-based catalysts compared to other promoters. Amvrosios et al. [100] prepared Ni/LnOx-type catalysts (Ln = La, Ce, Sm, or Pr) via a citrate sol-gel method and evaluated the dry reforming of methane. It was found that despite the high oxygen storage capacity and redox properties of CeO₂ [101,102], the Ni/CeO₂ catalyst showed lower CH₄ and CO₂ conversion values (just 13% and 35%, respectively) at the conditions of 750 °C, Ar/CH₄/CO₂ = 2.5:1.5:1, and WGHSV = 200,000 mL g⁻¹ h⁻¹, a much lower H₂/CO ratio between 0.3 and 0.6, and high amounts of coke deposits, making it the worst-performing catalyst. The specific surface area of the Ni/CeO₂ catalyst was 3 m² g⁻¹, while the specific surface area of the Ni/La₂O₃ catalyst was 15 m² g⁻¹. Hence, the Ni/La₂O₃ catalyst showed the highest reactant conversions, the highest H₂/CO ratios, and less pronounced coking. But the coke resulted in a rapid increase in the reactor back pressure. The pressure increased to very high values between the 12th and 13th hour, surpassing 3 atm at the 18th hour of the experiment. This indicated that the experiment could not be carried out far beyond the 18th hour. Khan et al. [103] found that the 10Ni-2La/γ-Al₂O₃ catalyst showed the highest initial reaction activities (~92%) and a lower deactivation rate (76%) with a mean deactivation rate of 0.53%/h and little observed coke formation (~2.23 mg coke/gcat·h) in 30 h reaction stream at 750 °C, while the deactivation rate of 10Ni-2Ce/γ-Al₂O₃ catalysts was fast from 82% to 48% with a mean deactivation rate of 1.13%/h and little observed coke formation (~2.4 mg coke/gcat·h). And, for 10Ni-2Co/γ-Al₂O₃, the activity was 74% to 34% with a mean deactivation rate of 1.33%/h and little observed coke formation (~3.1 mg coke/gcat·h) in the same conditions. The reason why the CH₄ conversion of the 10Ni-2La/γ-Al₂O₃ catalyst was almost 98% throughout the reaction duration of 30 h reaction was closely related to its resistance to coke formation, which can be attributed to the high surface area (27 m² g⁻¹), better Ni-La interaction with the support interface, and high stabilization of catalysts. Zhu et al. [104] investigated the CO₂ reforming of methane into synthesis gas over the Ni/SiO₂ catalysts promoted by La, Mg, Co, and Zn. They found that the deactivation of Ni-Co/SiO₂ catalysts might be mainly due to the sintering of metallic Ni. The addition of Mg influenced the reducibility and electronic properties of the catalysts and might have increased the basicity of the catalyst, increasing the conversions of CO₂ and CH₄ to 83% and 78.9%, respectively. However, the deactivation of the Ni-Mg/SiO₂ catalyst was due to the fact that the rate of carbon formation originated from CH₄ decomposition and/or the Boudouard reaction might be faster than that of carbon elimination. This resulted in considerable carbon accumulation over the catalyst and blocked the active center with time on stream. For the Ni-Zn/SiO₂ catalyst, due to the increased interaction between NiO and the support, the relatively small diameter of Ni after a 30 h reaction was observed in the SEM images, whereas the Ni-La/SiO₂ catalyst maintained higher activity and excellent stability.

The introduced La can react with H₂O and CO₂ to promote coke removal and the regeneration of active Ni. La has previously been shown to reduce carbon deposition on Ni catalysts [105] and to significantly improve the H₂ and CO selectivity [106] and the conversion of CO₂ and CH₄. This is probably due to the fact that La improves the distribution of active sites on the surface of Ni-based catalysts, making them more selective for CH₄ conversion and H₂ yield. Guo et al. [107] studied the effect of La₂O₃ loading on Ni/SiO₂ catalysts on the activity of steam reforming and found that the carbon conversion reached 98.8% and the H₂ yield reached 62.8%. This was mostly attributed to the fact that La₂O₃ promoted the dispersion of active components, reduced the average particle size of Ni, and generated a large number of oxygen vacancies in the catalysts. Ni promoted the cleavage of carbon bonds, and the oxygen vacancies contributed to the dissociation of H₂O. La₂O₃ was found to promote the gasification of coke [108]. Diao et al. [109] found that La enhanced the reduction of Ni²⁺, promoted the dispersion of Ni over the highly ordered mesoporous Ni-based catalysts, and significantly improved the CO₂ activation, resulting in CH₄ and CO₂ conversions of 78.6% and 85.2%, and CO and H₂ selectivities of 94.1% and 85.0%, respectively, for the 10Ni5LaAl. Kiani et al. [110] found that the 20Ni-5.78La/SBA16 catalyst showed a CH₄ conversion rate of 94.21%, a CO₂ conversion rate of 90.12%, a H₂ yield of 90.53%, and a H₂/CO molar ratio of 2.03 in the reforming of methane to produce value-added syngas, showing excellent catalytic performance at 700 °C. However, with the increasing amount of La, the deposited coke reacted with the lattice oxygen of La₂O₃ to form CO₂, which resulted in a decrease in the conversion of CO₂. Dan et al. [111] found that the H₂ production rate of the Ni/La-Zr catalyst was 15% higher than that of the other catalysts, approximately 60%, which was attributed to the fact that La accelerated the rate of the methane-reforming reaction and promoted the CH₄ decomposition to form H₂ and CO. Li et al. [112] found that NiLa/SiO₂ showed the best catalytic stability by decreasing the CH₄ conversion from 61.6 to 60.3% within 10 h. This indicated that La accelerated the oxidation of carbon on the Ni surface, which significantly increased the activation energies for CH₄ and CO₂ dissociation, as well as the reaction rate, leading to better anti-carbon performance. Appropriate La promoters provide more basic sites and oxygen vacancies, which facilitate carbon gasification [113]. The close contact between the Ni and La phases results in the formation of La₂O₂CO₃ intermediates, which inhibits carbon deposition and enhances the gasification of carbon by hydrocarbons (CH_x and carbon atoms) on the surface of Ni [114]. The strong interaction between the Ni and La phases results in less sintering of the metal [115]. The improved resistance to coking and sintering with La leads to enhanced catalyst stability, which is sustainable. Chen et al. [116] prepared Ni/La-Si catalysts with different La loadings for the reforming process and found that the conversion of CO₂ and CH₄ decreased from 75.9% and 73.2% to 62.8% and 60.4%, respectively, after 30 h for Ni/Si catalysts. The long-term stability of the Ni-based catalysts exhibited a marked improvement with the incorporation of the La promoter. Upon the introduction of 3.0 wt.% La, there was a slight decline in the CH₄ conversion, from 83.3% to 79.9%, and the selectivity of H₂ and CO was maintained at nearly 100% throughout the reforming process within 30 h. It was indicated that the appropriate La promoter effectively inhibited the H₂-consumption RWGS reaction [117] and significantly enhanced the catalytic activity and stability. Stability tests of the effects of La promoters on the Ni-based catalyst properties are presented in

Table 2. The time-on-stream stability tests of the effects of La promoters on the Ni-based catalyst properties.

Reaction Conditions	Time on Stream (h)	Effects	Reasons
CH4/CO2/H2O = 1:0.8:0.4; GHSV = 48,000 mL·h−1 g−1; atmospheric pressure	24 [118]	Ni/ZrO2: Both CH4 and CO2 conversions decreased from 90% to about 70%. La-Ni/ZrO2: Both CH4 and CO2 conversions were maintained above 90%; the selectivity of H2 and CO fluctuated up and down to 83% and 95%, respectively.	The La-Ni/ZrO2 catalyst had increased Ni dispersity, intensified Ni−support interaction, and enlarged oxygen vacancies, which led to excellent catalytic activities.
CH4/CO2 = 1:1; total flow rate = 40 mL/min; 750 °C; GHSV = 24,000 mL·h−1 g−1	34 [119]	The CH4 conversion of the Ni/5La-hydroxyapatite (HAP) dropped from 73.2 to 64.1% with a deactivation rate of 0.27%/h; H2/CO ratio: 0.84–0.81. The CH4 conversion of the Ni/HAP catalyst exhibited a rapid decline within 5 h, from 66.0 to 50.0% (the deactivation rate was 0.47%/h).	La enhanced the alkalinity and reinforced CO2 adsorption.
WHSV = 37,500 mL/(gcat·h); 800 °C; CH4/CO2/H2O/N2 = 1:0.4:0.8:0.6; 1 atm	10 [120]	The CH4 and CO2 conversions of NiLa5/MAO catalyst reached 93% and 71%, respectively, while the activity decreased by only 1% and 2.5%, respectively; The CH4 and CO2 conversions of Ni/MAO catalyst (85%, 63%) decreased by 5.6% and 8.4%, respectively.	La provided sufficient active sites and effectively controlled the metal sintering, thus leading to higher stability.
800 °C, 1 atm; GHSV = 1.584 × 105 mL/gcat·h; CH4/CO2/H2O = 1/0.4/0.8	60 [121]	The Ni/3.0La-Si catalyst activity was maintained at a relatively high level at 60 h: ~70% for CO2 and ~83% for CH4 and showed the smallest deactivation rate. The CO2 and CH4 conversions of the Ni/Si catalyst significantly declined to ~36% and ~40%, respectively.	La promoted the DRM reaction or suppressed the methane decomposition over a long time.
600 °C; H2O/CH4 = 3; GHSV = 32 × 103 mL/gcat·h	12 [122]	The CH4 conversion of Ni-3La/Al was the highest (about 90%).	The small active NiO sites were highly and homogenously dispersed on the support, thus exhibiting high resistance to sintering and coking.
750 °C; flow rate: 55 mL/min; CH4/CO2 = 1.2:1; GHSV = 33,000 mL/g·h	30 [123]	The CH4 conversion of the Ni-2La-SiO2 catalyst dropped from 60.2 to 55.9% with a deactivation rate of 0.14%/h; the CH4 conversion of the Ni/SiO2 catalyst decreased to 46.3% (the deactivation rate was 0.49%/h).	The strong basic sites ensured quick reactions between carbon and adsorbed CO2, hence resulting in obvious advantages in resistance to carbon deposition and sintering.
800 °C, CH4/CO2/O2 = 50/40/10; GHSV = 60,000 mL h−1 g−1	72 [124]	The Ni/La catalyst exhibited stable performance during 40 h of reaction; the CH4 and CO2 conversions decreased after 60 h of reaction.	La caused surface reconstruction and considerably enhanced the Ni species predisposed for catalytic activity and stability.

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Mo et al. [125] studied the catalytic activity and selectivity of Ni-based catalysts with different La loadings in a 150 h stability test and found that without La, the conversion of CH₄ decreased significantly from 86.11% to 80.95% in only 30 h, which indicated that the active components on the carrier were seriously deposited or sintered. On the other hand, the methane conversion of La-Ni-1 was almost maintained at 95%, showing high activity and excellent stability. As characterized by TG, the carbon deposition rate of the La-Ni-1 catalyst was only 1.63 mg/(h·gcat) in the 150 h stability test, which indicated that the rare-earth metal La had a significant advantage in preventing carbon accumulation [126]. Varun et al. [127] substituted La and Ni into nanocomposites by both impregnation and lattice substitution methods. The La/CZ (ionic) catalysts showed a hydrogen conversion of 55%, an increase in the oxidation states of La³⁺ and Ni³⁺, and a decrease in the Ni²⁺ sites. Compared with the alkali nanocomposites (CZ), the reforming reaction was more favorable and exhibited enhanced stability. This is mainly due to the enhanced H₂ dissociation with La. As the reforming reaction proceeds, La³⁺ enters the crystal lattice and generates more oxygen vacancies, and the adsorption and dissociation of oxygen molecules increase the number of active sites on the catalyst surface [128], thereby enhancing the catalyst’s performance. This further promotes the activation of H₂ and O₂ on the active sites (La³⁺) and oxygen vacancies. The reduction and oxidation cycles continued while the active sites were regenerated throughout the reaction process, which contributed to the sustainability of the catalyst activity and stability.

5. Conclusions and Outlook

To alleviate the carbon deposition and sintering problems of Ni-based catalysts in the reforming of methane, La promoters are considered to improve their catalytic activity. Compared with other metal additives, the Ni particles were the smallest in the Ni-based catalyst with La, indicating that the addition of La can promote the dispersion of the active component Ni, improve the stability of the Ni catalyst, and inhibit the high-temperature aggregation of Ni particles, thus improving the reactivity and anti-carbon properties of the catalyst. The additive La can react with the accumulated carbon, slow down the wrapping of the catalyst active sites by carbon, enhance the adsorption and activation ability of the catalyst, and improve the selectivity for CO and H₂ along with the conversion of CH₄ and CO₂. It was found that the addition of 2–5 wt.% La had significant effects on the activity and stability of the catalysts, as well as the chemisorption of CO₂.

However, it is a challenge to maintain the high degree of dispersion of Ni metal particles, due to the fact that the easy agglomeration of Ni at high temperatures is inevitable in the reaction. Since coke formation is limited at high temperatures, future research should be focused on the design of ideal Ni-based complex catalysts that can function at lower temperature ranges and provide considerable CO₂ and CH₄ conversion. Additionally, although La can alleviate the negative impacts of coking, the overall reforming effect will have a decreasing tendency over time. The carbon deposition may result in a rapid increase in the reactor pressure, which makes La enhance the cycle in a relatively short-time stability test during the reforming reaction. Hence, the overall service life and recycling mechanism of Ni-based catalysts with La promoters should be further studied, which can lower costs in industrial production and minimize negative impacts on the environment, in line with the requirements of sustainable development. Furthermore, the preparation methods of the modified Ni-based catalysts should be appropriate. The use of different preparation methods can modulate the morphology and crystalline phase of the Ni-based catalysts, which in turn affects the interaction with the carrier and active component, and thus the performance of the catalysts. Finally, from the perspective of safety, the Ni ions released from the Ni-based catalysts during the methane-reforming reaction are toxic. Hence, the release of Ni should be controlled as much as possible by means of optimizing the reaction conditions, thus reducing the impact on the environment and the operators.